1. Field of the Invention
The present invention relates to a magnetic impact tool for generating impact force through magnetism, and more specifically relates to a technique whereby a large magnetic impact action can be generated using a low-torque motor, and magnetic impact action can be performed even on a low-load screw in which the load torque does not exceed magnetic attraction torque.
2. Description of the Related Art
Conventional impact tools are used in operations in which machine screws are tightened with a bit such as a driver bit, and these tools comprises a hammer that is rotated with rotational force transmitted from a motor, an output shaft that meshes with the hammer and rotates, and a spring for urging the axially movable hammer toward the output shaft, wherein the output shaft is intermittently struck in the direction of hammer rotation and is rotated by means of a meshing portion between the hammer and output shaft. A bit mounted in the front end of the output shaft can thereby be rotated with impact (refer to Japanese Laid-open Patent Publication Hei 11-333742, for example).
However, considerable noise is generated with the impact tool cited in the above-described publication because the hammer and the output shaft directly collide with each other, and this noise is a severe drawback.
In view of the above, the present applicants have previously proposed a magnetic impact tool with which screws are tightened by using magnetic coupling to deliver a strike without any contact, and obtaining a tightening rotational impact force without a collision sound (not previously known). The magnetic impact tool (hereinafter referred to as tool) shown in FIGS. 15 to 17 is now explained as a premise of the present invention. The motor assembly 3 comprises a motor 31 and a reduction gear 32, and an output shaft 33 protrudes from the reduction gear 32. The magnetic hammer 2 comprises four magnets 21, eight pole plates 22 that sandwich the poles from each polar direction, and a cylindrical hammer core 20 having a pole attachment 23 with which the pole plates 22 are held on the peripheral surface. Each magnet 21 is sandwiched by two pole plates 22 and is bonded to the pole attachment 23.
A magnetic anvil 1 is provided that comprises an axle unit 11 that has a bit hole 12 for attaching a tool bit (not depicted) to the tip, and four L-shaped magnetic anvil arms 10. The magnetic anvil arms 10 are disposed at equal intervals along a circumference that is orthogonal to the axle unit 11, and are joined to the axle unit 11 about the center of the circumference. The motor assembly 3, magnetic hammer 2, and magnetic anvil 11 are housed in a case 6, as shown in FIG. 16. The hammer core 20 of the magnetic hammer 2 is inserted and fixed in a drive shaft 33. The magnetic anvil 1 is rotatably supported by bearings 4 in the tip portion of the case 6, and is also rotatably supported at the tip of the drive shaft 33 by hammer bearings 5, which are press-fitted in the center inside the L-shaped magnetic anvil arms 10. The L-shaped magnetic anvil arms 10 are positioned around the outside of the magnetic hammer 2. A grip 7 that is held by the operator is integrally formed on the case 6, and a trigger 8 for operating the magnetic impact tool is provided to the area where the index finger of the operator would be.
In its assembled state, the tool has a minute gap 9 between the magnetic anvil arms 10 and the magnets 21 or magnetic pole plates 22 of the magnetic hammer 2, as shown in FIG. 17, and the magnetic anvil arms 10 and magnetic hammer 2 can rotate without making contact. The operator can start the motor 31 and fasten screws by pulling the trigger 8 with the index finger.
The operation of the above-described tool entails inserting a screw-tightening bit (not depicted) into the bit hole 12 at the tip of the magnetic anvil 1, pressing the screw to the fastened member while causing the screw tightening bit to make contact with the screw, and pulling the trigger 8 to begin driving the tool. In the initial state of screw tightening, the magnetic hammer 2 and the magnetic anvil 1 synchronously rotate to perform screw tightening because the magnetic attraction torque that acts between the magnetic hammer 2 and the magnetic anvil 1 is greater than the torque required for screw tightening. When the torque required for screw tightening is greater than the magnetic attraction torque of the magnetic hammer 2 and magnetic anvil 1, the torque of the motor assembly 3 and the inertia torque are applied, the magnetic hammer 2 and magnetic anvil 1 are not synchronized any longer, a magnetic impact condition is established, and screw tightening is completed with impact torque.